1. Field of Invention
The present invention generally relates to a device and method for digitizing an analog signal, and more particularly to a device and method for digitizing a receiver signal strength indicator (RSSI) signal in a wireless transmission system.
2. Description of Related Art
When a wireless transmission system, such as a mobile phone system, is in operation, transmission and receiving of signals are generally affected by the surrounding environment. To indicate the quality or strength of a received signal, a receiver signal strength indicator (RSSI) signal is typically generated from a transceiver of the wireless transmission system. Various operations or control modes are performed based on the RSSI signal indicated.
In another aspect, micro-controllers are becoming very popular nowadays, thanks to the advancements of semiconductor technologies. The functionality and speed of micro-controllers are significantly enhanced, while the size is significantly reduced. Furthermore, the power consumption of the micro-controllers are also reduced. It is no wonder that micro-controller are widely adopted as a controller in the wireless transmission system.
Basically, the RSSI signal from a transceiver is an analog signal. A digitization process needs to be performed for the RSSI signal before sending it to the micro-controller for processing. To digitize the RSSI signal into a digital one, a conventional analog to digital converter (ADC) can be used as shown in FIG. 1, where a wireless transmission system 100 includes a transceiver 110, a controller 120, and an ADC 130. The transceiver 110 is used to receive and transmit a radio frequency signal Srf, and output a RSSI signal Vsi as an indication of the signal strength. The controller 120, for example, a single chip micro-controller, is responsible for controlling operations of the wireless transmission system 100. The ADC 130 is used to digitize the analog RSSI signal Vsi into a digital signal Vsi', which is then used by the controller 220.
Although the above-mentioned method using an ADC to digitize the RSSI signal is convenient and straightforward, the cost, however, is higher. To reduce the cost, another conventional method as shown in FIG. 2 was proposed, in which a wireless transmission system 200 includes a transceiver 210, a controller 220, a comparator 231, and a digital to analog converter (DAC) 232. During the digitalization process, a numeric control signal SD from the controller 220 is sent to the DAC 232. A threshold voltage V.sub.th, which is proportional to the numeric control signal SD is then generated from the DAC 232 as a comparison basis with the RSSI signal Vsi. When the RSSI signal Vsi is larger than the threshold voltage V.sub.th, the numeric control signal SD is increased so that the threshold voltage V.sub.th is accordingly increased. The RSSI signal Vsi is then compared with the threshold voltage V.sub.th again. If the RSSi signal Vsi is still larger than the threshold voltage V.sub.th, the above-mentioned process is repeated until the threshold voltage V.sub.th is larger than the RSSI signal Vsi, where the numeric control signal SD represents the digitized RSSI signal Vsi. As shown in FIG. 3, the threshold voltage V.sub.th with an increasing variation from a smallest value is depicted. The digitization process is not completed until the threshold voltage V.sub.th is larger than the RSSI signal Vsi at t1.
Although a DAC circuit using switches and resistors can be used instead of using the commercial DAC chip when accuracy and cost are not major concerns for digitizing the RSSI signal. However, the circuit complexities involved are significantly increased. The controller also needs extra output pins for sending the numeric control signal to the DAC. Furthermore, conventional digitization circuits constructed by using either ADC or DAC components have a fixed level (or step) for quantization, which is not flexible.
As a summary, the above-mentioned device for digitizing the RSSI signal has the following, disadvantages:
1. The costs are higher if the available commercial ADC or DAC components are used to construct the device.
2. The circuit complexities and quantities of components are increased if the self-designed DAC circuit using switches and resistors is used, which consequently increase the costs as well.
3. The conventional digitization methods have a fixed quantization level, which is not flexible.